Which of these numbers is prime? ${29,\ 33,\ 51,\ 65,\ 70}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 29 are 1 and 29. The factors of 33 are 1, 3, 11, and 33. The factors of 51 are 1, 3, 17, and 51. The factors of 65 are 1, 5, 13, and 65. The factors of 70 are 1, 2, 5, 7, 10, 14, 35, and 70. Thus, 29 is a prime number.